Nicholas Steno's anatomical drawing of an extant shark (left) and a fossil shark tooth (right). Steno made the leap and declared that the fossil teeth indeed came from the mouths of once-living sharks.

The fossil record provides snapshots of the past that, when assembled, illustrate a panorama of evolutionary change over the past four billion years. The picture may be smudged in places and may have bits missing, but fossil evidence clearly shows that life is old and has changed over time.

Early fossil discoveries
In the 17th century, Nicholas Steno shook the world of science, noting the similarity between shark teeth and the rocks commonly known as "tongue stones." This was our first understanding that fossils were a record of past life.

Two centuries later, Mary Ann Mantell picked up a tooth, which her husband Gideon thought
to be of a large iguana, but it turned out to be the tooth of a dinosaur, Iguanodon.
This discovery sent the powerful message that many fossils represented forms
of life that are no longer with us today.

Additional clues from fossils
Today we may take fossils for granted, but we continue to learn from them. Each new fossil contains additional clues that increase our understanding of life's history and help us to answer questions about their evolutionary story. Examples include:

Indication of interactionsThis ammonite fossil (see right) shows punctures that some scientists have interpreted as the bite mark of a mosasaur, a type of predatory marine reptile that lived at the same time as the ammonite. Damage to the ammonite has been correlated to the shapes and capabilities of mosasaur teeth and jaws. Others have argued that the holes were created by limpets that attached to the ammonite. Researchers examine ammonite fossils, as well as mosasaur fossils and the behaviors of limpets, in order to explore these hypotheses.

Clues at the cellular level
Fossils can tell us about growth patterns in ancient animals. The picture at right is a cross-section through a sub-adult thigh bone of the duckbill dinosaur Maiasaura. The white spaces show that there were lots of blood vessels running through the bone, which indicates that it was a fast-growing bone. The black wavy horizontal line in mid-picture is a growth line, reflecting a seasonal pause in the animal's growth.